1. Field of the Invention
This invention relates to a system for implanting ions into an object such as a semiconductor wafer.
2. Description of the Related Art
An ion implantation system is used to implant ions as impurities into semiconductor wafers in the process of manufacturing semiconductor devices. In such an ion implantation system, ions drawn out of an ion source unit are made to pass through a mass analyzing magnet and an acceleration tube and then implanted into a semiconductor wafer while they are scanning in X- and Y-directions through an electrostatic deflector plate to produce a given pattern. As the ion source unit of the system, a Freeman type ion source unit where source gas is turned into plasma and ions are drawn out of the plasma by means of an ion extraction electrode is popularly used.
An ion source unit of the above described type comprises an arc chamber where a magnetic field is produced to "tighten" the swell of the plasma in it so that an intense ion beam may be drawn. The intensity of magnetic field in the unit is kept constant (and not greater than 150 to 200 Gauss) regardless of the type of ions involved. Because, if the magnetic field is made too intense for the swell of the plasma to be tightened, the swell is away from the ion extraction slit of the unit and consequently the efficiency of the ion extraction operation is lowered. It is obviously conceivable, however, that keeping the intensity of magnetic field constant in the unit may be disadvantageous in some cases depending on the type of ions involved and reduce the efficiency of the ion extraction operation.